1. Field of the Invention
The present invention relates to a photomask, a photomask set, an exposure apparatus, and an exposure method.
2. Description of the Related Art
Regarding flexible printed circuits (hereinafter referred to as circuit boards or substrates) used in electronic devices or the like, demands for densification are increasing along with miniaturization and enhanced functionality of electronic devices. To correspond to such densification, there have been attempts to densify a substrate by forming a multi-layer circuit board in which conductive layers of the substrate are provided on both faces or in three or more layers. As one way of densification, a separate substrate which connects multi-layer printed circuit boards, on which various types of electronic parts are mounted, via a connector or the like is widely used mainly in small electronic apparatuses such as lap-top personal computers, digital cameras, cell phones, game machines, and the like.
Incidentally, when it is attempted to densify a substrate having conductive layers at least on its front and rear faces, that is, having two or more layers, it is important that the positional accuracy of a conductive pattern formed on each layer or each of the front and rear faces is high. Specifically, when the positional accuracy of the conductive pattern on each layer or each of the front and rear faces is high, it is possible to reduce the diameters of lands of inter-layer connection parts, and there is a merit to suppress positional displacement of the conductive patterns on the front and rear faces when a target in the subsequent process is disposed only on one face. In connection with increasing the positional accuracy of such a conductive pattern formed on each layer or each of the front and rear faces, there are technical ideas disclosed in Patent Documents 1 to 4.
FIG. 13 is a cross-sectional view schematically illustrating an exposure method of Patent Document 1. As illustrated in FIG. 13, the method of Patent Document 1 performs alignment by reading with a CCD camera a ring-shaped alignment mark added to a substrate and a circular alignment mark added to a photomask. This method is basically able to perform alignment only on one face at a time.
FIG. 14 is a cross-sectional view schematically illustrating an exposure method of Patent Document 2. As illustrated in FIG. 14, the method of Patent Document 2 provides through holes in a substrate, and provides different alignment marks on each of photomasks of a front side and a rear side. Then, the through holes and the alignment marks of the photomasks of the front side and the rear side are aligned, to thereby align the substrate and the photomasks on the front side and the rear side.
Further, Patent Document 3 discloses a method using vacuum suction for a work retaining means. Moreover, Patent Document 4 discloses a work retaining means of clamping type.    [Patent Document 1] Japanese Patent No. 4921789    [Patent Document 2] Japanese Patent Application Laid-open No. 2007-121425    [Patent Document 3] Japanese Patent No. 4218418    [Patent Document 4] Japanese Examined Patent Application Publication No. H5-41982
Now, in the method disclosed in Patent Document 1, when photosensitive resists formed respectively on both front and rear faces of the substrate are exposed, it is necessary to align each of the front face and the rear face of the substrate with the photomask on the front side. Consequently, positional displacement occurs two times between the ring-shaped alignment mark of the substrate and the circular alignment mark of the photomask, and thus it is difficult to improve alignment accuracy for the front and rear faces of the substrate.
Further, in the method disclosed in Patent Document 2, concavities and convexities exist between a copper foil and a transparent layer (for example, a polyimide film) of the substrate for increasing adhesiveness. Consequently, when the through holes are formed by removing the copper foil of the substrate through rear surface processing, there is a problem that light scatters through the concavities and convexities existing on the transparent layer, and edges of the through holes and edges of the alignment marks existing on the photomasks on the rear side become unclear, which decreases the alignment accuracy. Further, when the through holes are formed by boring the substrate through machining, there is a problem that the alignment accuracy decreases due to occurrence of variation in processing accuracy. Moreover, through the method disclosed in Patent Document 2, the substrate is pulled off a roll winding part to an exposure area, and the substrate is retained with tension at the exposure area. Consequently, there is a concern that the substrate can be stretched, and also it is difficult to improve the alignment accuracy because retention at the exposure area is not stable.
Note that when the work retaining means of Patent Documents 3, 4 are applied to the methods disclosed in Patent Documents 1, 2, it is difficult to improve the alignment accuracy between the photomasks and the substrate.
The present invention is made based on the above-described situation, and an object thereof is to provide a photomask, a photomask set, an exposure apparatus, and an exposure method which are capable of improving the alignment accuracy between a substrate and respective photomasks disposed on the front and rear faces of the substrate.